As is known, in the process of transformation of most plastic materials into granules or granular materials, a very important treatment consists of the process of dehumidification of the plastic material granules, i.e. the removal of the water contained in the granules of those polymer materials which are hygroscopic.
The elimination of the moisture from the hygroscopic granules is necessary since the granules themselves must be subjected (during the transformation steps) to melting at relatively high temperature; during such steps, the residual water molecules contained in the granules can be inserted into the molecular chains of the polymers and cause their breakage. If the molecular chains should break, as will be understood, poor quality final products would be obtained; in any case, products would be obtained with inadequate mechanical characteristics, considering that bubbles, blowholes, coloration non-uniformity and other undesired phenomena are often generated, ascribable to the presence of water in the starting granules.
The granular plastic materials to be subjected to dehumidification are typically stored in fluid tight hoppers connected to a generator device of hot and dry air, commonly called a “dryer”, which is intended to feed or insufflate hot and dry air, so-called process air, into the hoppers.
Once it has entered into the hopper, the process air crosses all or part of the mass of plastic material granules to be dehumidified, removing the moisture contained therein, and then it exits from the hopper through a suitable outlet duct.
In a dehumidification process in a hopper as described above, the degree of attainable dehumidification for a given granular plastic material depends on many factors, such as the residence time of the granular plastic material inside the dehumidification hopper, the flow rate of hot and dry air through the hopper, the treatment temperature and the thermo-fluid-dynamic behavior in the interaction between the air current and the granular plastic material, the latter also depending on the internal geometry of the hopper.
For each granular plastic material to be dehumidified, the dehumidification degree required or to be attained is established, or in any case it is established at the start of the dehumidification process. For such dehumidification degree, the residence time interval of the granules in the hopper is established, as well as the process temperature, so as to allow the diffusion of the water molecules from the granule interior to outside the granules.
As is known, the phenomenon of the diffusion of water molecules inside the granular material is governed by the equation of Fick's second law:
            ⅆ      C              ⅆ      t        =      D    ⁢                            ⅆ          2                ⁢        C                    ⅆ                  X          2                    
where C is the concentration of the water molecules, which depends on the time t and on the spatial coordinates X of the granule, and D is the diffusion coefficient of the moisture in the specific plastic granular material.
The diffusion coefficient, in turn, depends on other variables according to the Arrhenius model:
  D  =            D      0        ⁢          exp      ⁡              (                              -            U                    kT                )            
in which D0 is a diffusion constant which depends on the type of material to be treated, U is the activation energy, k is the Boltzmann constant and T is the temperature in Kelvin.
It will be understood, therefore, that obtaining and maintaining a uniform temperature in the granules residing in a dehumidification hopper is of fundamental importance, at least in the radial direction, i.e. in the granules at the same cross section of the dehumidification hopper.
If, instead, the granules had different temperatures in radial direction, there would be different diffusion constants D, and different dehumidification degrees would be obtained in the granules, which would negatively affect the characteristics of the manufactured article or final product.
Furthermore, according to the analysis of Fick's second law, one infers that it is of fundamental importance that the residence time of the granules of one same material in a hopper is nearly the same. Different residence times in a hopper in fact lead to different moisture concentrations in the granules exiting from the dehumidification hopper.
If the granules of a same material must remain for the same residence time in a hopper, i.e. in the step of descent of the granular material into the dehumidification hopper, this means that the vertical components of the velocity field of the single granules must remain substantially constant over an entire cross section of the hopper.
It should be recalled that the flow that is established during the descent of the granular material inside the hopper can be mainly of two types: “mass flow” and “funnel flow”.
The “mass flow” flow type is characterized by uniform descent of the material inside the hopper, i.e. the modules of the various velocity vectors along a straight section of the hopper will be similar; in other words, the granular material descends in a uniform manner and preferential descent channels are not formed therein.
On the other hand, the “funnel flow” flow type has a discontinuity in the values of the vector modules along a cross-section of the hopper. The velocity vectors in the central part of the hopper along a same cross-section will have a greater value than that of the modules of the velocity vectors close to the walls of the hopper. This leads to the formation of a preferential descent channel of the material in the central part of the hopper, which naturally occurs for the hopper structures lacking inserts, like those described hereinbelow.
In many applications, typically in the field of plastic material working, it is essential that a “mass flow” type behavior of the granular material be obtained in the descent step in the hopper.
Solutions have already been proposed that provide for prearranging, in a dehumidification hopper, a hollow insert composed of a body with constant cross section terminating on its lower part with a conical portion. One such solution, which allows obtaining a mass flow, is for example taught in the European patent application EP-2 090 856 in the name of the same applicant of the present application, where it is provided that the hollow interior of the insert is in fluid communication with the outlet of a dryer and the lower conical portion of the insert is perforated or pierced, such that by feeding air from the dryer to the insert, pressurized process air is fed at the conical portion of each insert into the granules loaded in the hopper.
With one such solution, a uniform descent is obtained of the granular material inside the hopper, and the residence time of the plastic material granules to be treated is substantially the same for all the granules. Nevertheless, with one such hopper structure, in the plastic material granules located at a same level, i.e. in the radial direction, one obtains an undesired thermal gradient, which can involve non-uniform levels of dehumidification of the granules, i.e. granules are obtained having different moisture levels, which can compromise the qualitative characteristics of the final manufactured product.
U.S. Pat. No. 3,875,683 teaches a hopper for dehumidifying plastic material granules, which is suitable for keeping the dehumidification air at a predetermined and constant temperature. Such a hopper includes a main body having an upper cylindrical section, and a lower tapered portion; the main body further includes an upper loading mouth for loading plastic material granules, and a discharge opening for the dehumidified material granules.
An insert member is located inside the main body, thereby delimiting therewith an annular space for the material to be dehumidified. A plurality of holes is formed on the wall of the insert member.
The lower tapered portion of the main body includes two walls, one external wall enclosing an internal wall, in such a way as to delimit a chamber external to the annular space, and in fluid communication therewith, and to this end a number of holes is formed on the internal wall of the external chamber. The external chamber is put in fluid communication with the internal part of the insert member by means of ducts.
Moreover, the hopper structure comprises heating means located inside the insert member and the external chamber. Optionally, further heating means can be located adjacent to an upper wall of the main body.
With such a hopper, the dehumidified gas is supplied to the chamber, passes partly through the holes formed on the internal wall thereof, and partly is fed to the inner zone of the insert (by means of the above-mentioned ducts), and crosses the respective holes, thereby entering the annular space.
Owing to such a structure, the material granules are crossed by a stream of dehumidified gas having the same temperature for the whole height of the hopper. Thus, the exhaust gas, after having crossed the material to be dehumidified, has a temperature very close to the temperature at which it has been supplied to the hopper.
U.S. Pat. No. 3,875,683 aims at devising a hopper in which the dehumidification gas temperature is kept constant in the whole annular space containing the material to be dehumidified.
In plants for working plastic material granules, the granules should not be subjected to thermal stress, otherwise their rheological properties can be negatively affected. Therefore, in the hopper, it is fundamental to obtain a correct thermal gradient along the vertical axis of the hopper, and not a constant temperature as obtainable with the solution taught by U.S. Pat. No. 3,875,683.
Moreover, with the hopper taught by such US patent, a satisfying dehumidification with a reduced energy consumption cannot be obtained.
In this regard, it should be noted that the gas discharged from the hopper has the same (high) temperature of the gas supplied into the hopper, and thus it should be cooled before being regenerated, since, as it is known, the regeneration is carried out at low temperatures, e.g. a temperature lower than 40°.
FR-2 674 944 A1 teaches an apparatus for dehumidifying plastic material granules including a hopper having a lower tapered section, a duct tangentially connected to the lower tapered section, and a fan for supplying gaseous fluid into the duct. The apparatus further includes a conical insert member, located in the hopper and having an apex turned upward, the insert member delimiting with the hopper an annular space for the material to be dehumidified.
The lower tapered section of the hopper has two walls, one enclosing the other, thereby delimiting a chamber. A number of holes are formed throughout the whole length of the internal wall of the lower tapered section. The dehumidifying air is supplied to the tangential duct, and then to the external chamber, from which it enters and rises along the annular space, thereby crossing the granular material to be dehumidified.